The laying of asphalt paving material on road surfaces entails spreading an aggregate-filled tar-based paving material on a prepared road bed. The paving material is spread while hot and is then compacted so that upon cooling a hard pavement surface is formed. Conventional paving machines utilize a heavy metal plate termed a "screed," usually constructed of steel or iron, to compact the paving material. The screed is typically mounted on pivot arms at the rear end of the paving machine, and "floats" up and down in the vertical direction as it is pulled over the paving material. The weight of the screed, as well as other structures carried on the screed, acts to compress and tamp the paving material into a compact layer.
To facilitate compaction of the paving material, the screed must be heated, typically to a temperature of about 180.degree. to 300.degree. F., to assist the paving material in flowing under the screed and to reduce adhesion of the paving material to the screed. If the screed is not adequately heated, the tar in contact with the bottom of the screed begins to harden, resulting in buildup of paving material and excessive drag.
Most conventional screeds are heated through the use of fossil fuel powered burners that heat the upper surface of the screed by the direct application of flame or hot exhaust gases. For example, many conventional screed heaters are powered with propane gas or fuel oil. The use of fossil fuel burners to heat screeds has several drawbacks. Combustion of fossil fuels generates large amounts of smoke, particularly when fuel oils are burned.. This smoke represents a source of environmental pollution, and may also pose a potential health hazard to paving workers. Additionally, because the burners heat some portions of the screed, where the flame or exhaust gas actually contacts the screed surface, more than other portions of the screed, warping of the screed may result. The contour of the screed determines the evenness of the paving material that is being compacted by the screed. Screeds are often flexed under extreme tensile loads during use to achieve desired crowning or other surface contour of the paving material. When a screed has been warped by the uneven application of heat, the contour produced by the screed may be other than that which is desired.
Most conventional floating screeds are also subjected to vibration during use to help in compaction of the paving material. For example, eccentric cams are mounted in contact with the screed and are rotated to induce vibration. Any heating systems for screeds must be able to withstand this high frequency vibration without breakage or failure.
One alternate heating system that represents an improvement in the environmental drawbacks of fossil fuel powered screed heaters is disclosed in U.S. Pat. No. 5,096,331 to Raymond. That patent discloses a "liquid circuit heat transfer system" for heating screeds, and entails forcing hydraulic oils through a narrow orifice. Friction created by flow of the oil through the orifice results in an increase in temperature of the oil, which is routed through a tubing system to heat the screed. While offering some advantages over fossil fuel powered heating systems, such liquid circuit heating systems suffer from other drawbacks. Because of the vibration to which the screed is subjected, there is a potential for breakage of tubes or fittings through which the heated oil is routed, which could result in leakage of the heating fluid. Additionally, the time required to initially bring the screed to the temperature upon start of operation may be unduly long due to the limited heat capacity of the heating fluid.
Also, conventional liquid current heating systems utilize hydraulic fluid as the heating fluid. The heating fluid circuit is typically connected to the paving machine's primary hydraulic circuit. This connection results in the inability to utilize the primary hydraulic circuit to power other hydraulic devices included on the paving machine while the heating system is being operated. For example, the vibratory units on paving screeds are typically powered by the hydraulic pump on the paving machine. When a liquid circuit heat transfer as conventionally configured is utilized, it is not possible to simultaneously operate both the heat system and the vibratory units.